The present invention relates to the art of phase-controlling AC electric current or power which is applied to a load circuit having resistive and inductive components. The invention finds particular application in phase control for resistance welding and will be described with particular reference thereto. It is to be appreciated that the invention is also applicable to phase control in conjunction with other resistive-inductive loads such as motor controls, resistance heating, incandescent lighting, and the like.
A resistance welding apparatus commonly receives an AC electrical voltage. A contactor selectively gates portions of the current in each half line cycle to a welding load. Commonly, the welding load includes a welding transformer which is connected with a pair of electrodes that are clamped on either side of the workpiece to be welded. In resistance welding, the current of the AC power lags the voltage by a determinable phase angle. Because the contactor can only conduct current to the load in one direction at a time, the contactor cannot start conducting current to the load in one half cycle until the current from the other half cycle is extinguished. The maximum weld power is achieved by firing the contactor to gate current in each half cycle immediately upon extinction of the current in the preceding half cycle. Under this maximum power operating condition, the weld system is governed by the equations: ##EQU1##
In the equations, P is the maximum available weld power, E is the weld voltage, I is the weld current, .phi. is the power factor angle, R is the resistance of the overall system including the workpiece being welded, j is the 90.degree. operator, w is the frequency of the electric power, and L is the inductance of the overall system including the workpiece. The power factor angle is a system constant which is determined by the inductance and resistance of the contactor, the load circuit including the welding transformer, the welding electrodes, the workpiece, and the like.
To maintain the equations independent of the AC electric power, each line cycle is commonly described as having a duration of 360.degree.. For 60 Hz. electric power, each degree is just under 50 microseconds in duration. To reduce the weld power or temperature, the firing angle, i.e., the angle or duration into each half line cycle at which the contactor is fired, can be retarded beyond the power factor angle. When the firing angle is retarded, less energy is stored in the inductive components of the circuit reducing the hangover angle, i.e., the angle or duration which the current pulse extends into the next cycle before it is extinguished.
In a phase-controlled system, it is known that the root mean square (RMS) value of the actual weld current is determined by the equation: ##EQU2## Here, and in addition to the above-noted designations, a is the firing angle, b is the hangover angle, t denotes time, and e is the natural logarithm. It is further known that there is a fixed relationship between the firing angle a, the hangover angle b, and the power factor angle .phi., which is governed by the equation: ##EQU3## Thus, if two of the angles are known, the third can be calculated. The solutions for equations (4) and (5) are generally not determinable by classical techniques, but can be solved using iterative techniques.
Heretofore, many welding machines have been constructed with a first manual adjustment to adjust for the inherent power factor angle of the welding system and a second manual adjustment, commonly denoted as a percent heat adjustment, for adjusting the firing angle to select a percentage of the maximum available weld heat or current. Frequent recalibration of the power factor adjustment was required to maintain the weld heat constant. Mushrooming of the electrodes, variation in the thickness of the workpieces, differences in the surface properties of the workpieces, and the like would change the power factor angle of the system. The power factor calibration was also used to avoid saturation of the core of the welding transformer.
To avoid the frequent power factor recalibration, welding controls were developed which attempted to provide an automatic power factor correction. Many of these schemes were based upon determining the average power factor of the system and its variation over a plurality of preceding complete welds. Based on this average and variation in the power factor, the automatic control would project an anticipated power factor for the next complete weld or several welds. These automatic power factor control schemes had varying degrees of success but most led to the introduction of other problems into the control scheme.
To make the power factor correction more accurate, it has been proposed to base the power factor correction on the first cycle of the weld in progress. Specifically, this scheme included firing the first half cycle of the weld at a preselected firing angle and measuring the resultant hangover angle. The hangover angle from the first half cycle indexed or addressed a look-up table of data which related power factor to hangover angle for the preselected constant firing angle. The power factor from the look-up table was used to make the power factor adjustment in all subsequent half line cycles of the weld.
One of the problems with this power factor correction scheme is that it requires the first half cycle to be fired at the preselected firing angle which may be inappropriate for the selected weld. Further, if the subsequent cycles of the weld are to be much lower than the preselected first half cycle firing angle, the welding transformer may become overexcited with flux and interfere with proper control of the subsequent low current line cycles. A second problem is that the power factor does not necessarily remain constant over all cycles of the weld. During the weld, the circuit resistance is increased by the heating of the weld current conductors and the workpiece. The inductance is altered by the strong magnetic fields generated during the weld and by distortion of the conductors caused by the strong magnetic fields.
Another problem with the prior art welding controls resides in the fact that they are unsuited to preventing welding transformer core saturation during pulsation welding. In pulsation welding, the weld current is applied for several line cycles, then the contactor blocks the weld current for several line cycles, then the contactor again conducts the weld current for several line cycles, and so forth. This alternating conduction and nonconduction pattern may be carried out several times. Because the prior art devices do not control the first half cycle weld current in each group of welding pulses, the percent heat must be controlled separately for each first half cycle to prevent core saturation.
Still another problem with prior welding controls is that fluctuations in the RMS voltage received by the welding apparatus cause similar fluctuations in the maximum available weld current. This, in turn, causes fluctuations in the actual weld heat produced by a given percent heat setting. This failure to control the actual RMS weld current accurately diminishes the quality of the weld.
The present invention overcomes the foregoing problems and others to provide a welding apparatus and method for accurately controlling the RMS value of the weld current in every half line cycle of the weld.